Yartek's life's ambition was to take control of the Conscience of Marinus in order to control the people of Marinus. (PROSE: Doctor Who and the Keys of Marinus) He was, for a time, given counsel by the Monk, who worked as his advisor. (PROSE: No Future)
Seven hundred years after the improvement of the Conscience of Marinus, Yartek invented immunisers for himself and the Voords, (PROSE: Doctor Who and the Keys of Marinus) allowing them to rob, murder and exploit the people of Marinus. To prevent him from gaining control of the Conscience, the key microcircuits were removed and hidden. (TV: The Keys of Marinus)
Years later, the Voords killed Arbitan and Yartek declared himself to be in control. He captured Altos and Sabetha, taking the keys from them and exploiting their feelings for each other to force Altos to tell him where the fourth key was. Learning that the First Doctor and his companions had it, Yartek posed as Arbitan, wearing his hood and pretending to be afflicted by a disease, to acquire it from Ian Chesterton and Barbara Wright.
Yartek inserted what he believed to be the final key into the Conscience, unaware that it was a fake, (TV: The Keys of Marinus) and prepared himself to finally achieve his dream and ascend to godhood. (PROSE: Doctor Who and the Keys of Marinus) He was killed in the resulting explosion, (TV: The Keys of Marinus) his flesh seared to the bone in a millionth of a second and his bones turning to ash. (PROSE: Doctor Who and the Keys of Marinus)
While pondering whether he should try to find out the Enemy's true identity, the Eighth Doctor once reflected that the Enemy's identity would probably be dreadfully disappointing, and cited the Enemy being "Yartek, leader of the alien Voord, carrying a big stick" as one such guess. (PROSE: The Taking of Planet 5)